Water, especially in the western United States and other arid regions, is a valuable resource. Many oil and natural gas production operations generate, in addition to the desired hydrocarbon products, large quantities of waste water, referred to as “produced water”. Produced water is typically contaminated with significant concentrations of chemicals and substances requiring that it be disposed of or treated before it can be reused or discharged to the environment. Produced water includes natural contaminants that come from the subsurface environment, such as hydrocarbons from the oil- or gas-bearing strata and inorganic salts. Produced water may also include man-made contaminants, such as drilling mud, “frac flow back water” that includes spent fracturing fluids including polymers and inorganic cross-linking agents, polymer breaking agents, friction reduction chemicals, and artificial lubricants. These contaminants are injected into the wells as part of the drilling and production processes and recovered as contaminants in the produced water.
Commonly encountered non-natural contaminants in produced water, and their sources, are discussed below.
From high-viscosity fracturing operations—gellants in the form of polymers with hydroxyl groups, such as guar gum or modified guar-based polymers; cross-linking agents including borate-based cross-linkers; non-emulsifiers; and sulfate-based gel breakers in the form of oxidizing agents such as ammonium persulfate.
From drilling fluid treatments—acids and caustics such as soda ash, calcium carbonate, sodium hydroxide and magnesium hydroxide; bactericides; defoamers; emulsifiers; filtrate reducers; shale control inhibitors; deicers including methanol and thinners and dispersants.
From slickwater fracturing operations—viscosity reducing agents such as polymers of acrylamide.
Because of the very wide range of contaminant species as well as the different quality of produced water from different sources, efforts to create a cost effective treatment system that can treat or recycle the spectrum of possible produced water streams have little success. For example, while reverse osmosis is effective in treating many of the expected contaminants in produced water, it is not very effective in removing methanol and it may be fouled by even trace amounts of acrylamide.
As another example, there have been many attempts to reclaim produced water and reuse it as fracturing feed water, commonly referred to as “frac water.” Frac water is a term that refers to water suitable for use in the creation of fracturing (frac) gels which are used in hydraulic fracturing operations. Frac gels are created by combining frac water with a polymer, such as guar gum, and in some applications a cross-linker, typically borate-based, to form a fluid that gels upon hydration of the polymer. Several chemical additives generally will be added to the frac gel to form a treatment fluid specifically designed for the anticipated wellbore, reservoir and operating conditions.
However, some waste water streams are unsuitable for use as frac water in that they require excessive amounts of polymer or more to generate the high-viscosity frac gel. For example, trace amounts of spent friction reducers in the stream inhibit the added polymer from gelling. Because it can be difficult to prevent produced water streams from different sources from being co-mingled, this typically results in all produced water from a well field being made unsuitable for recycling as frac water.
An additional problem occurs when the produced water is also contaminated with methanol and it is desirable to discharge the water to the environment. One way to treat produced water to the extent necessary to discharge the water to the environment, is through filtration techniques such as ultra filtration and reverse osmosis. However, methanol will pass through nearly any available membrane filtration technology.
Yet another problem occurs when the produced water is also contaminated with boron, such as from the use of borate-based cross-linking agents, and it is desirable to discharge the water to the environment. One way to treat produced water with boron is referred to as the HERO® process in which the pH is raised up to at least about 11 prior to treatment with reverse osmosis, resulting in the boron being rejected with the reverse osmosis reject brine. However, raising the pH has several undesirable attributes. First, there is increased scaling within the reverse osmosis system increasing the maintenance costs of the system. Second, the pH must then be reduced before the treated water may be discharged to the environment. Third, the cost of the chemicals to raise the pH coupled with the cost of immediately thereafter lowering the pH and the cost of disposal of the precipitated salts resulting from the lowering of the pH make the HERO process very expensive.